July 15, 2013

Researchers Solve Mystery Of Ancient Algal Bloom

An ancient bloom of life that occurred just after the last ice age isn't what it seemed at first, according to new research in the journal Nature Geoscience.

Scientists had long believed that the bloom, which occurred around 14,000 years ago, was spurred by rising sea levels washing iron into the ocean. However, the new study suggested that a "perfect storm" of conditions allowed for the bloom to happen, and not simply the abundance of iron.

"A lot of people have put a lot of faith into iron - and, in fact, as a modern ocean chemist, I've built my career on the importance of iron - but it may not always have been as important as we think," said co-author Phoebe Lam, an associate scientist at Woods Hole Oceanographic Institution (WHOI).

The study results have important implications for the ongoing debate over using geoengineering techniques to combat carbon emissions, specifically seeding the oceans with iron in an attempt to create a massive algae bloom that would absorb carbon dioxide from the atmosphere and sequester it deep in the ocean.

While previous studies on the ancient bloom looked at evidence from the ocean floor, the team from the WHOI and other institutions decided to look for signs of iron on the continental shelf that might be responsible for seeding a massive bloom.

To search for clues, Lam and her colleagues examined a sediment core taken near Russia's Kamchatka Peninsula. They sampled the core about every 2 inches, moving back in time as they sampled deeper along the core, to the time period before the bloom began. The samples were analyzed for chemical composition, explicitly checking for signature neodymium and strontium isotopes that would indicate which kind of iron was present. The isotope ratios allowed the team to determine where the iron came from - the ancient Loess Plateau of northern China or younger, volcanic sources.

Instead of finding out where the massive iron influx came from, the team said they were surprised to find evidence of steadily declining iron levels.

"We saw the flux of iron was really high during glacial times, and that it dropped during deglaciation," Lam said. "We didn't see any evidence of a pulse of iron right before this productivity peak."

According to the research team, warming conditions caused deep water mixing in the North Pacific. The resulting upwelling of nutrients also brought surface plankton into deeper, darker waters. Glacial melt then released a burst of freshwater - stopping the mixing and trapping the plankton in a layer of bright, nutrient-rich ocean water.

"We think that ultimately this is what caused the productivity peak - that all these things happened all at once," Lam says. "And it was a transient thing, because the iron continued to drop and eventually the nutrients ran out."

In addition to solving an ancient mystery, the study's findings call into question the idea of simply using iron to reduce atmospheric carbon.

"This study shows how there are multiple controls on ocean phytoplankton blooms, not just iron," said Ken Buesseler, a WHOI marine chemist who was not among the study's authors. "Certainly before we think about adding iron to the ocean to sequester carbon as a geoengineering tool, we should encourage studies like this of natural systems where the conditions of adding iron, or not, on longer and larger time scales have already been done for us and we can study the consequences."